Signal propagation delay is a major component of a system timing equation. For common clock signals, propagation delay directly determines a maximum operating frequency, whereas the delay between the data and a strobe is critical for Source Synchronous (SS) signals. The propagation delay depends on board parameters such as the dielectric constant of mother board materials, line thickness, and the distance to neighboring conducting lines. These variables vary from one board to another as a result of non-zero manufacturing tolerances. Characterizing the delay is important to understand the system margin. Thus, one finds it necessary to develop a simple and reliable non-intrusive method to determine signal propagation delay between circuits.
One method for determining signal propagation delay involves using an oscilloscope and two probes to measure the amount of delay. The amount of delay is read from the oscilloscope display. This method is manual, tedious and labor intensive and slow. In addition, the method can be used only to the extent that the points to be probed are exposed on the board. Also, probe tip capacitance can load a lightly loaded circuit and alter the signal timing.
Another method involves a counter and a filter. The filter extracts a high frequency component of the signal to be measured. The extracted high frequency component charges a capacitor. A comparator compares the level of charge on the capacitor to a reference level. A signal from the comparator when the reference level is exceeded causes a counter to stop counting. The count of the counter, when counting stops, is proportional to signal propagation delay. This method is impractical in general since the method requires a filter and counter to be built into the receiving circuitry of each of the circuits between which the signal travels. Also, for accuracy, the capacitor and filter components must be compensated for voltage and temperature changes.